This invention relates to a switch circuit connected between a signal feed line and a reference potential (e.g. ground potential) for operatively short-circuiting the signal feed line to the reference potential line to interrupt the transmission of a signal through the signal feed line. More particularly, the present invention relates to a monolithic semiconductor integrated switch circuit suitable for operatively interrupting the transmission of a signal containing an a.c. signal of high voltage amplitude.
An audio tape recorder or a video tape recorder usually has a single magnetic head which is used in common for recording and for playback, with a recording switch and a playback switch which are used to change over the recording mode and the playback mode. The magnetic head has a first head terminal to which a recording amplifier and an a.c. bias oscillator as well as the playback switch are connected, and a second head terminal connected to a playback amplifier and the recording switch. In the playback mode, the first head terminal is grounded by turning the playback switch on. Thereby, a signal to be recorded from the recording amplifier and an a.c. bias signal from the bias oscillator are not supplied to the head. A signal recorded on a recording tape is played back via the head. In the recording mode, the second head terminal is grounded by turning the recording switch on, thereby to prohibit the playback signal output. A signal to be recorded and the a.c. bias signal are supplied to the head. In this manner, the two switches are interposed between the two head terminals and the ground line to change over the operation modes. For these switches, electronic switches including transistors are used, because mechanical switches are not reliable and generate chattering noises. Especially, bipolar transistors are used for the electronic switches because of their advantages of a small series resistance and a low residual voltage.
In the prior art, the collector and emitter of a bipolar transistor as the switching transistor are connected to the head terminal and to the ground, respectively. A switch-control signal is applied to the base of the switching transistor to control its conductivity. With respect to the switching transistor for the playback switch, the a.c. bias signal from the bias oscillator and the signal to be recorded are applied to its collector during the recording mode. Since the collector of the switching transistor in a monolithic integrated circuit device having the conventional P-N junction isolation structure must be isolated from a semiconductor substrate by the P-N junction, a voltage lower than the minimum voltage of the a.c. bias signal must be applied to the semiconductor substrate in order to maintain the reverse-bias to the P-N junction between the semiconductor substrate and the collector of the switching transistor to which the a.c. bias signal is applied. This also means that a large voltage higher than the peak-to-peak voltage of the a.c. bias signal is applied across the P-N junction between the semiconductor substrate and the collector. For these reasons, the integrated circuit device must be provided with a bias voltage source for generating the low voltage to be applied to the substrate and with a particular structure to enhance a breakdown voltage of the reverse-biased P-N junction higher than the above-mentioned large voltage. This is very disadvantageous, as compared with a normal integrated circuit device in which the semiconductor substrate is grounded and the collector of a transistor isolated from the substrate via a P-N junction receives a potential of one polarity.